1. Field of the Invention
The present invention relates to a method for preparing a light-emitting diode having a vertical structure, and more particularly to a method for preparing a light-emitting diode having a vertical structure by stripping GaN base epitaxial layer and sapphire substrate by a wet process.
2. Description of the Related Art
A light-emitting diode is a light-emitting device which converts the electrical energy to the light energy and is currently the most promising next-generation light source. The light-emitting diode usually gives out light through intrinsic transitions of direct band gap semiconductor and has a high photoelectric conversion efficiency, namely internal quantum effect.
The luminous efficiency of the GaN base light-emitting diode is significantly improved in recently years, but it cannot completely replace the conventional light resource. At present, the quantum efficiency, the current distribution uniformity and device cooling capacity has become main technical bottlenecks which limit further improvements of the performance of light-emitting diodes.
Semiconductor light-emitting diodes can be divided into two categories according to geometry structures: semiconductor light-emitting diodes with lateral structure and semiconductor light-emitting diodes with vertical structure.
The main disadvantages of the semiconductor light-emitting diodes with lateral structure include low heat dissipation efficiency, current congestion, low current density and high production cost. In order to resolve the heat dissipation problem of the semiconductor light-emitting diodes with lateral structure, flip chip technology is provided. However, the flip chip technology is very complicated and the production cost is high.
In addition, the sapphire which is usually used as a support substrate of a light-emitting diode has drawbacks that the electrical conductivity is very low and its lattice constant does not match with the GaN material. The thermal resistant of the device is increased due to the very low electrical conductivity, which causes serious self-heating effect. Moreover, the mismatch of the lattice constant between the sapphire and the GaN material leads to a high defection density in the GaN epitaxial layer and also results in a strong biaxial stress existing in the GaN. Especially, when the temperature of the chip is changing, the difference of thermal expansion coefficients will bring additional thermal stress into the GaN.
At present, a lot of researches are carried out for the semiconductor light-emitting diodes with vertical structure. Two electrodes of the semiconductor light-emitting diode with vertical structure are located on two sides of the epitaxial layer, which has the same advantage of high heat dissipation efficiency as flip chip technology and does not have disadvantages of flip chip technology, and also has advantages such as uniform current distribution, increased current density, and high light extraction efficiency and so on.
However, during the preparing process of the semiconductor light-emitting diode, the growth substrate is usually stripped by laser or mechanical grinding. The production cost for stripping the growth substrate by laser is high, and the laser beam causes damage to the epitaxial layer, therefore resulting in a low yield. For the method which strips the growth substrate by mechanical grinding, since the precision of the mechanical grinding is low and the grinding has non-uniformity, the grinding precision cannot be controlled and the products yield is low.
Therefore, there is a need to provide a method for preparing a light-emitting diode having a vertical structure without stripping the growth substrate by non laser or mechanical grinding method.